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Gregg A. Snedden

Publications and source records attributed to Gregg A. Snedden.

At least 19 recordsLinked to original sources
USGS2026

Ecological transformations of coastal wetlands of the conterminous United States in response to contemporaneous sea-level rise

Coastal wetlands are among the most important ecosystems on the planet but are increasingly imperiled by accelerating sea-level rise (SLR). In the past, biogeomorphic feedbacks have allowed coastal wetlands to adjust vertically and persist through periods of accelerated SLR. Recent rates of SLR are faster than any in recent geologic history, making the fate of coastal wetlands highly uncertain. Here, we synthesize surface elevation table marker-horizon data from 442 stations in coastal wetlands across the conterminous United States to evaluate if they are largely persisting in the face of accelerated SLR, or if they are undergoing ecological transformations of submergence and/or migration. Across the conterminous United States, 11% of coastal wetlands in this sample are on a trajectory of submergence whereas 73% of sites are lagging SLR, but may be able to migrate upslope, and 16% of sites are gaining elevation at rates which exceed SLR indicating persistence and an ability to migrate seaward. Vulnerability of these systems to ecological transformation varies across the three coasts of the conterminous United States which, span large biogeomorphic gradients. These results serve as one of the first national syntheses of coastal wetland elevation trends and may help focus conservation and restoration efforts in a rapidly changing future.

conterminous United States↗
USGS2026

High and dry: The persistence of diverse coastal marsh communities depends on available upland area and connection to surrounding waters

Sea-level-rise and alterations to natural hydrology have contributed to coastal land loss. To combat coastal land loss and maintain the ecosystem services provided by marshes, marsh creation has been broadly implemented. Marsh creation projects can be performed in conjunction with the construction of elevated terrestrial ridges that seek to mimic historic delta-formed distributary ridges. Ridges and similar elevated structures, such as those surrounding created marshes, offer the opportunity to sample the ecotones between low marsh and upland areas within a small area. We sampled marsh assemblages on elevation gradients at eight sites in the Louisiana coastal zone. We identified nine marsh communities based on co-occurrence of taxa and abiotic conditions (soil salinity, percent time inundated, soil percent moisture, soil bulk density, and soil pH) with a two-step process of hierarchical clustering and random forest classification. Inundation time and salinity were identified as the most important conditions related to community composition. We used future estimates of coastal elevations based on local relative sea-level-rise estimates to make predictions of community change up to 50 years into the future. Our predictions, meant to serve as generalizations for the coastal region, indicate a near total loss of high-elevation salt marsh communities at the sites sampled. Elevation variation supports diverse marsh communities when water is allowed to flow as in a natural marsh. Hydrology regulates inundation, and by extension, salinity. Evaluating hydrologic exchange can be incorporated into decisions regarding future marsh creation to support marsh health and persistence.

Louisiana↗
USGS2026

Local, regional, and coastwide effects of interactions between storms and relative position in tidal frame on Louisiana coastal marshes

Context In coastal wetlands, land area is dynamic in both space and time, and it is crucial to understand the factors that may tip the balance between wetland area loss and gain. Flooding has been shown to be a key regulator of wetland resilience to land loss through effects on vegetation health and productivity. However, the relationship between elevation and land change is complex, likely interacting with lateral processes such as edge erosion to influence land change rates. Objectives We sought to determine the role of storms, major drivers of lateral erosion, to help explain the complex relationships between elevation and land change at multiple spatial scales and improve predictions of wetland loss. Methods We used long-term records of elevation change, water elevation, and surface wind stress together with remotely sensed land-area change datasets to determine the factors that contribute to land loss in Louisiana (USA) coastal wetlands. Results Our results illustrate that wetland elevation alone cannot be used as a predictor of land change. Annual time-integrated wind stress, which we used as an indicator of storminess, is an important predictor of land change and interacts with elevation to impact wetland gain or loss. The data showed high site-level variation, indicating that local factors strongly determine land change. Nonetheless, we were able to identify broad generalities at the larger scales. We found that the interaction of flooding and storminess varied among geographic regions along the coast. In the Delta Plain region of the coast, sites that are frequently flooded (low elevation) were more likely to experience land gain in stormy years and land loss in calm years. Conversely, sites that were frequently drained (high elevation) had greater land loss during stormy years, presumably due to wind-wave erosion. This trend was not observed in the heavily managed Chenier Plain region, where perpetually flooded wetlands gained land during stormy years, likely due to wind-driven drainage of the marsh. Conclusion Together, these results illustrate that models of wetland vulnerability based upon elevation change or storm effects alone are too simple and will not capture observed trends in land change. Incorporating additional factors such as wind stress and management status can improve predictions of coastal wetland loss and our understanding of the mechanisms controlling resilience of these ecosystems.

Louisiana↗
USGS2025

Seasonal variation in bay-marsh sediment exchange through a back-barrier salt marsh tidal creek

Salt marsh resilience to sea-level rise largely depends on the balance of sediment exchanges with surrounding bays. In this study, we investigate mechanisms that determine residual sediment fluxes using continuous measurements of bay-marsh sediment exchange conducted in a tidal creek spanning 13 months (753 tidal cycles) in an intertidal marsh recently subsidized with sediment via thin-layer placement. The maximum water level in each tidal cycle varied over seasonal and fortnightly timescales and was driven by a combination of the seasonal cycle in mean sea level (maximum in September, minimum in January) and the fortnightly spring-neap cycle. Residual water fluxes tended to be ebb-directed during overbank tides, possibly due to water crossing creekshed boundaries in the intertidal zone when water levels were sufficiently high. Sediment concentrations on the ebb of overbank tides exceeded those of their corresponding floods, but only for tidal cycles in which water temperatures exceeded 14°C. The interaction of these dynamics resulted in over 90% of the net sediment export from the creek occurring during overbank tides during warmer months—conditions met in 30% of the observed tidal cycles. These findings exemplify the importance of accounting for seasonality in sediment fluxes when assessing sediment budgets of salt marshes and illustrate how sediment budgets assessed with shorter duration datasets may exhibit seasonal bias. Additionally, they suggest that sediment retention for thin-layer sediment placement projects may be high over the course of the first year after sediment subsidies are deployed.

New Jersey↗
USGS2025

Time-varying rates of organic and inorganic mass accumulation in southeast Louisiana marshes: Relationships to sea-level anomalies and tropical storms

Louisiana's coastal wetlands are complex systems that require a continuous input of organic and inorganic material to keep pace with relative sea-level rise. Coastal restoration projects such as sediment diversions are being implemented to mitigate land loss and increase availability of inorganic sediment to coastal wetlands, and marshes specifically rely on organic material to build soil volume and maintain surface elevation. Interannual-to-decadal sea-level anomalies such as hurricanes can affect marsh accretion, mineral deposition, and plant productivity. In this light, complex ecogeomorphic feedback controls whether a marsh surface is sustainable or eroded/drowns. This study performs some of the first differential vertical accretion rates (VARs) and organic and inorganic mass accumulation rates (MARs) over time in SE Louisiana marshes determined from the 210 Pb Constant Rate of Supply model, coupled with standard 137 Cs VARs. These accumulation rates over the past ∼100 years were measured from a total of six brackish and salt marsh locations in Barataria Basin near the proposed Mid-Barataria Sediment Diversion. They were then related to interannual sea-surface elevations at Grand Isle, Louisiana, over the last ∼60 years and recorded hurricane activity in the delta. Results show VARs range from 0.63 cm/y to 1.69 cm/y and total MARs range from 0.11 to 0.43 g/cm 2 /y. Temporally, VARs and MARs (total, inorganic, and organic) are characterized by gradual increases in rates with decreasing age along with episodic peaks in VARs and MARs. The findings of this study indicate that no relationship occurs between sea-level anomalies and VARs or organic and inorganic MARs; however, a strong relationship appears to occur between major hurricanes to VAR and MAR contributions. Furthermore, high water content (81 ± 8%) and organic-rich soils in the sediment cores highlight the significance of belowground biomass and associated pore volume in maintaining marsh elevation in the study area.

Louisiana↗
USGS2024

Topographic and bathymetric survey in support of the effectiveness assessment of the living shoreline restoration in Gandys Beach, New Jersey

High resolution topobathymetric field surveys were conducted by the U.S. Geological Survey in collaboration with Northeastern University and in cooperation with the U.S. Fish and Wildlife Service and The Nature Conservancy in a selected shoreline along Gandys Beach, New Jersey, from January to April 2018. These data are a critical model input for hydrodynamic and wave models and can affect the accuracy of model outputs such as wave height, water surface elevation, current velocity, and sediment transport. Gandys Beach is a living shoreline where constructed oyster reefs (CORs) were built to protect the shoreline and enhance habitat for oyster and other species. Because of the complex topography and bathymetry of the study area, higher spatial resolution topobathymetric data are required to resolve the vertical variations near the CORs. During the field survey, the global navigation satellite system positioning method was used to establish the elevation of a benchmark referenced to the North American Vertical Datum of 1988. The topobathymetric data were collected using a total station. Horizontal accuracy of plus or minus 0.05 foot (ft) and vertical accuracy of plus or minus 0.10 ft were calculated using root mean square error between duplicate surveys. Two existing datasets were integrated with the survey data to create an updated topobathymetric dataset for model input and analysis: (1) the U.S. Geological Survey Coastal National Elevation Database 1-meter resolution data developed after Hurricane Sandy and (2) The Nature Conservancy 2017 elevation monitoring data at 10-meter resolution. A root mean square error analysis comparing survey data with the new topobathymetric dataset versus the survey data compared to the original Coastal National Elevation Data dataset showed errors of 0.31 and 2.61 ft, respectively. This improved dataset can be used for wave and hydrodynamic modeling in support of the effectiveness assessment of the CORs and living shoreline restoration along Gandys Beach.

New Jersey↗
USGS2024

Monitoring of wave, current, and sediment dynamics along the Fog Point Living Shoreline, Glenn Martin National Wildlife Refuge, Maryland

Living shorelines with salt marsh species, rock breakwaters, and sand nourishment were built along the coastal areas in the Glenn Martin National Wildlife Refuge, Maryland, in 2016 in response to Hurricane Sandy (2012). The Fog Point living shoreline at Glenn Martin National Wildlife Refuge was designed with the “headland - breakwater - embayment” pattern. Scientists from the U.S. Geological Survey, Northeastern University, U.S. Fish and Wildlife Service, and Louisiana State University studied wave, current, and sediment dynamics to assess the effectiveness of the Fog Point living shoreline structures in terms of wave attenuation and erosion reduction. Wave gages, current meters, sediment traps, sediment tiles, and lateral erosion pins were deployed along the Fog Point shoreline during February 10–14, 2020. Because of COVID-19 pandemic travel restrictions, sensors were not retrieved until August 25, 2021, which was 18 months after field deployment, resulting in tremendous loss or damage of sensors and sediment measurements. Monitoring data indicated that wave heights were substantially reduced at locations behind the breakwater (headland) compared to the wave heights in the offshore location, but not at the location in the control area (the embayment). Current patterns and current velocities at the location behind the breakwater were complex and changed dramatically compared to the current patterns and current velocities offshore. Sediments were blocked by the breakwater most of the time except during periods of storms with wave heights larger than 0.9 meter, when waves overtopped the breakwater and brought sediments to the tidal flat and salt marshes behind the breakwater. Behind the breakwater, both sediment deposition and erosion were observed during the 18 months of monitoring. Continued low elevation marsh edge erosion from wave undercutting along the embayment was observed, especially at the existing wave-cut gullies. Monitoring results indicate that the “breakwater + marsh planting” structure along the Fog Point shoreline has limited shoreline protection capacity. Marsh edge erosion behind the breakwater was likely caused by the limited sediment supply from marine sources for transport and delivery, as well as the effects of circulation and current velocity on the settling and deposition of suspended sediments from eroded marshes. Marsh edge erosion continued in the embayment or control area where no shoreline restoration structures were implemented. Long-term (decadal scale) monitoring and adaptive management of living shoreline structures could help to assess the effectiveness of wave attenuation for reducing shoreline erosion and enhancing vegetation growth for trapping sediments and the effectiveness of marsh surface elevation growth for keeping pace with sea level rise.

Maryland↗
USGS2024

Patterns of marsh surface accretion rates along salinity and hydroperiod gradients between active and inactive coastal deltaic floodplains

High subsidence rates are inherent to coastal deltas worldwide, contributing to rapid rates of relative sea-level rise and compromising the sustainability of coastal wetlands. Different parts of river deltas, however, experience accretion or erosion, depending on the coupling between ecological and morphological processes. Wetland expansion occurs in active deltaic coastal basins that are connected to riverine sedimentation. In contrast, wetland degradation occurs in inactive deltaic coastal basins where river engineering strategies associated with flood control restrict river connectivity. Here, we investigated the relative role of inorganic and organic loading to marsh accretion rates spanning fresh to brackish to saline zones between active and inactive coastal deltaic floodplains of the Mississippi River Delta. Marsh surface accretion rates monitored over 36 months using the feldspar marker horizon technique ranged from 1.24 ± 0.35 cm yr −1 in the freshwater marsh to 2.94 ± 0.51 cm yr −1 in the saline marsh in the inactive coastal basin compared to an opposite trend in the active coastal basin with a low vertical accretion rate in the saline site at 1.12 ± 0.17 cm yr −1 and higher accretion values at the freshwater site (2.14 ± 0.49 cm yr −1 ). Our results suggest that saline marshes have high resilience identified by high vertical accretion rates exceeding those of river-dominated freshwater marshes in active deltaic floodplains. Overall, the marsh surface accretionary patterns detected in this study underscores the relative contribution of organic and inorganic sediments to elevation capital across salinity gradients between active and inactive basins in coastal Louisiana with particular interest to river management and restoration strategies. These findings, however, are applicable to coastal deltaic floodplains elsewhere given the repetition geomorphic forcings (e.g., relative contribution of riverine, tidal and wave power) and coastal typologies worldwide.

Estuarine, Coastal, and Shelf Science↗
USGS2023

Spatial variability in vertical accretion and carbon sequestration in salt marsh soils of an urban estuary

Salt marshes in New York City’s Jamaica Bay have been disappearing and deteriorating since early 1900s, resulting in the loss of long-term accumulated carbon storage. However, the spatial variations and mechanisms in vertical accretion and soil organic carbon (SOC) sequestration across this highly urbanized estuary remains unclear. In this study, we collected soil cores to a depth of ~ 50 cm across Jamaica Bay to study the spatial variability in long-term (50–100 years) vertical accretion, the accumulation of mineral sediment and organic matter, and SOC sequestration. Results of gamma spectrometry analysis of 137 Cs and 210 Pb show that there was moderate spatial variability in long-term vertical accretion rates across Jamaica Bay study sites (mean: 0.48 ± 0.13 cm yr − 1 , range: 0.36–0.78 cm yr − 1 ). This local scale spatial variability in vertical accretion is largely driven by spatial variations of sedimentation. The magnitude of the long-term vertical accretion in Jamaica Bay is significantly correlated with organic matter accumulation, but not with mineral sediment. However, the role of organic matter in contributing to vertical accretion has been declining. The declining role of organic matter to vertical accretion is reflected by the lower SOC sequestration rate (mean: 128 and range: 26–189 g C m − 2 yr − 1 using the 210 Pb dating technique) compared to the global mean salt marsh SOC sequestration rate (244 g C m − 2 yr − 1 ). This is especially so on the marsh islands in the degrading western part of the bay where SOC sequestration was less than half the global average.

New York↗
USGS2023

Monitoring of wave, current, and sediment dynamics along the Chincoteague living shoreline, Virginia

Nature-based features, also called living shorelines, are increasingly applied in coastal protection and restoration. However, the processes and mechanisms (feedbacks and interactions) of wave attenuation, current velocity change, and sediment deposition and erosion along the living shoreline remain unclear, thus limiting the adaptive management of living shoreline restoration projects for coastal shoreline resilience under future storm conditions. In this study, wave, current, and sediment dynamics along the Little Toms Cove living shoreline, Chincoteague National Wildlife Refuge, Virginia, a low wave energy environment, were investigated during a 2-month winter period in 2019 to examine the effects of living shoreline structures on shoreline protection and oyster habitat enhancement. It was found that wave attenuation by the living shoreline structures (oyster castles or constructed oyster reefs) is dependent on water depth, wind speed, wind direction, and local bathymetry. Analysis of observed data indicate that the oyster castles along the Little Toms Cove living shoreline play a limited role in wave attenuation in this low wave energy environment. During the 2-month winter period, wave energy was attenuated by 39.7 percent when oyster castles were emergent or slightly submerged with southwest winds. In contrast, when the oyster castles were fully submerged, wave energy behind the oyster castles increased by 38.6 percent. The construction of oyster castles affected circulation patterns with increase or decrease in velocity at nearshore waters protected by the castles depending on locations of measurements in relation to the oyster castles. Bottom shear stress analysis indicates that tidal currents play a larger role than waves on shoreline and marsh edge erosion along the Little Toms Cove shoreline during the 2 months of field monitoring. The oyster castles protecting the marsh edge and tidal flat from erosion resulted in higher fine sediment concentration in the water column landward of the castles because more sediment was retained in the lee side of the castles. It is important to maximize sediment within the wetlands and adjacent mudflats behind the oyster castles. Erosion from the marsh edge and interior serves as the major source of sediment for this wetland system due to the limited sediment supply in Assateague Channel. Furthermore, it was found that the oyster castles along the Little Toms Cove living shoreline were inundated more than 60 percent of the time, leading to the enhanced oyster habitat as evidenced by suitable velocity (less than 10 centimeters per second) and mean grain size (less than 0.08 millimeters) for oyster feeding and the increased oyster shell density and growth in the intertidal zone protected by the castles than in the control area. More field data (for example, concurrent monitoring of sediment concentration and salinity) over other seasons (for example, summer) could help examine the long term and combined engineering and ecological benefits of living shoreline restoration projects under seasonal and enhanced future storm conditions.

Virginia↗
USGS2023

Field observations and long short-term memory modeling of spectral wave evolution at living shorelines in Chesapeake Bay, USA

Living shorelines as a nature-based solution for climate change adaptation were constructed in many places around the world. The success of this type of projects requires long-term monitoring for adaptive management. The paper presents a novel framework leveraging scientific machine learning methods for accurate and rapid prediction of long-term hydrodynamic forcing impacting living shorelines using short-term measurements of water levels and wind waves in the largest estuary in the U.S. Different from existing data-driven wave prediction models focusing on significant wave heights, this study is focused on the prediction of wave energy spectra in shallow water using winds and tides as the input feature and short-term measurements of wave spectra and water depths as the label. Long Short-Term Memory (LSTM) models were developed using four-month wave measurements in the stormy seasons to predict integral wave parameters and energy spectra for multiple years. The developed models accurately predicted wave heights, peak periods, and energy spectra around the living shorelines, capturing complex wave dynamics, such as wave generation by wind, nonlinear wave-wave interactions, and depth-limited wave breaking in the shallow water of a large estuary. The validated models were then used to determine the long-term wave forcing impacting the living shorelines based on the modeled wave characteristics and spectra. Model results show that the surrogate models utilizing LSTM to predict wave spectra in the frequency domain enable long-term predictions of spectral wave evolution with a minimal computational cost. Our findings provide valuable insights into the efficacy of living shorelines in attenuating wave energy and demonstrate the utility of this approach in assessing the effectiveness of such living shoreline structures.

Applied Ocean Research↗
USGS2023

Modeling surface wave dynamics in upper Delaware Bay with living shorelines

Living shorelines gain increasing attention because they stabilize shorelines and reduce erosion. This study leverages physics-based models and bagged regression tree (BRT) machine learning algorithm to simulate wave dynamics at a living shoreline composed of constructed oyster reefs (CORs) in upper Delaware Bay. The physics-based models consist of coupled Delft3D-FLOW and SWAN in four-level nested domains. The model accuracy converges with increasing mesh resolution. The simulated wave-induced current circulation substantiates the effectiveness of CORs in trapping sediments. The simulated yearly-averaged wave power correlates qualitatively with historical shoreline retreat rates. BRT is adopted to improve the model accuracy, identify key processes responsible for simulation errors in wave height ( H 8 ) and wave period ( T p ), and quantify their importance. In the CORs sheltered area, BRT reveals that simulation errors of wind seas mainly arise from wind forcing, wave breaking and wave triad interactions. Wave breaking is seven times more important than wind forcing for simulating H 8 , while wind forcing and triad interactions are of equal importance for simulating T p . Simulation errors of swells mostly stem from bottom friction and offshore wave boundary conditions. Results from this study can help the assessment and adaptive management of CORs-based living shoreline restoration projects under climate change.

New Jersey↗
USGS2023

Sources and chemical stability of soil organic carbon in natural and created coastal marshes of Louisiana

Coastal marshes are globally important for sequestering carbon, yet sea-level rise and anthropogenic stressors can reduce their capacity as carbon sinks. Marsh restoration can offset a portion of carbon loss through the degradation of natural marshes, but potential differences in the sources and stability of soil organic carbon (SOC) between created and natural marshes may affect their function as a long-term carbon sink. Here, we examine the sources and chemical stability of SOC in natural and created marshes across the Gulf coast of Louisiana, USA. Marshes were examined along an estuarine salinity gradient in a former interdistributary basin of the Mississippi River Delta and in six created marshes across a 32-year chronosequence and a natural reference marsh ( n = 6) in the Chenier Plain. Carbon source was assessed using δ 13 C analysis and chemical stability was determined through an acid hydrolysis digestion that removed labile carbon (LC). Soil δ 13 C values suggested that the local vegetation dominated SOC in all natural marshes although brackish marshes had a mix of sources and degradation of SOC. Recalcitrant carbon (RC) was 72.2 ± 0.5 % of SOC across fresh, brackish and saline marshes. The depth-averaged RC accumulation rate was almost three times greater than LC accumulation rate, yet both contributed significantly to accretion and long-term SOC accumulation (124–132 g m −2 y −1 in natural marshes). RC and SOC accumulation rate increased with mineral sediment accumulation rate. For the created marshes, SOC became increasingly recalcitrant due to an increase in in-situ plant inputs, but accumulation rates were lower than the natural marshes. Overall, this study illustrates that natural marshes have a large stock of RC from the vegetation while dredge sediment created marshes have no plant-derived carbon initially, which accumulates slowly thereafter. Restoration practices may be improved by preserving and augmenting these deteriorating but carbon-rich natural marshes.

Louisiana↗
USGS2022

Biomass allocation of tidal freshwater marsh species in response to natural and manipulated hydroperiod in coastal deltaic floodplains

Deltaic floodplains are highly vulnerable to relative sea level rise (RSLR) depending on the sediment supply from river channels that provides elevation capital as adaptation mechanism. In river channels where levees have restricted sediment supply to coastal deltaic floodplains, river sediment diversions have been proposed as a restoration strategy to increase elevation allowing for marshes to establish and cope with RSLR. The response of coastal wetlands to surface elevation has been well-defined for estuarine marshes, but models for coastal deltaic floodplain marshes have not been resolved. Here we coupled field observations from biomass plots and a mesocosm experiment (‘marsh organ’) with remote sensing techniques to assess biomass allocation of tidal freshwater marsh species in response to gradients in hydroperiod in Wax Lake Delta (WLD), coastal Louisiana, U.S.A.. We found that, contrary to salt-tolerant species, Colocasia esculenta aboveground biomass (AGB) is strongly positively correlated with percent inundated time (R2 = 0.79, P < 0.001), increasing from (mean ± 1SE) 186 ± 69 g/m2 in the supratidal zone to 1422 ± 148 g/m2 beyond its natural occurrence range in the lower intertidal zone. Belowground biomass consistently exceeded AGB at 2363 ± 294 g/m2 on average across elevation treatments. We also found that C. esculenta expanded its surface coverage area by 31% in five years consistent with the growth and emergence of WLD's subaqueous platforms, reflecting this species ability to cope with higher inundation time. In contrast to earlier studies conducted in brackish and saline settings, where longer hydroperiods had negative effects on biomass accumulation, our data suggest that tidal freshwater marshes can cope with longer hydroperiods caused by river sediment diversions.

Estuarine, Coastal and Shelf Science↗
USGS2022

Estuarine Geomorphology, Circulation, and Mixing

To understand the processes affecting the distribution and cycles of particulates, pollutants, nutrients, and organisms in estuaries, it is insufficient to focus solely on the biological and chemical aspects of the processes. Water sources and movements (e.g. evaporation, precipitation, riverine discharge, submarine ground water discharge, wetland hydrology, and tidal exchange) as well as other hydrodynamic aspects of coastal systems, including circulation patterns, stratification, mixing and flushing, must also be considered. When hydrodynamic changes occur quickly relative to biological, geological, and chemical transformations, they become the dominant controlling factors of many ecological processes in estuaries (Officer 1980), and it is now widely recognized that a thorough understanding of the marine estuarine ecology requires comprehensive knowledge and integration of physical processes affecting the system. Using the terminology of a shallow-water oceanographer, this chapter aims to organize, classify, and describe some of these important physical characteristics and processes.

Book chapter↗
USGS2020

2023 Coastal master plan: Model improvement plan, ICM-wetlands, vegetation, and soil

As part of the model improvement effort for the 2023 Coastal Master Plan, the wetland processes captured by the morphology and vegetation models used during previous master plans were reevaluated to assess how Integrated Compartment Model (ICM) subroutines could be improved. This process considered technical reviews, comments, and suggested improvements provided by model developers, advisory groups, and other experts during previous master plan cycles. The availability of new data and information that could be used to make model improvements was also considered. In many cases, the team considered and tested multiple options or approaches. As a result of this effort, recommended improvements are provided here. The improvements recommended to be included in the 2023 Coastal Master Plan include: adjusting marsh collapse thresholds, refining organic matter accretion calculations, developing an unstructured grid for modeling vegetation, improving flotant marsh and forested wetlands algorithms, creating and applying an updated map of existing vegetation, adjusting model code, and updating the submerged aquatic vegetation (SAV) module. This report describes the team’s work through a series of 7 distinct activities to identify and test options for model improvements to ensure the updated ICM used for the 2023 Coastal Master Plan appropriately captures ecological and morphological processes observed in Coastal Louisiana. As appropriate, relevant literature and data are discussed. Test runs to evaluate how changes influence model outputs are also documented. A final list of recommended updates, taking into account consideration of all options and results from test runs, is summarized at the end of the report. A later report will describe the final ICM-LAVegMod and ICM-Morph subroutines for the 2023 Coastal Master Plan, detailing the updates that have been incorporated.

Louisiana↗
USGS2019

Patterning emergent marsh vegetation assemblages in coastal Louisiana, USA, with unsupervised artificial neural networks

Question Are self‐organizing maps (SOMs) useful for patterning coastal wetland vegetation communities? Do SOMs provide robust alternatives to traditional classification methods, particularly when underlying species response functions are unknown or difficult to approximate, or when a need exists to continuously classify new samples obtained under ongoing long‐term ecosystem monitoring programs as they become available?. Location Coastal Louisiana, USA. Methods A SOM was trained from in situ observations of 559 vegetation species relative cover data from 2526 samples collected over eight years at 343 locations across coastal Louisiana. Hierarchical cluster analysis was applied to the SOM output to delineate vegetation community types, and indicator species analysis was conducted. Salinity and flood duration were compared across the delineated community types. Results The SOM patterned the 2526 training samples into 260 output neurons, which were further clustered into eleven community types. Clear gradients in salinity and flood duration existed among the community types, and geographic zonation of the communities was evident across the landscape. At some locations assemblages were temporally stable; at other locations they varied considerably. Samples not used in training the network were effectively projected onto the SOM and assigned to one of the delineated community types. Conclusions The SOM was effective in delineating plant communities in the region that were qualitatively similar to those obtained in previous investigations. Being robust to skewed distributions and the presence of outliers, SOMs provide an alternative to traditional distribution‐based statistical approaches. Their ability to efficiently classify new data into existing community types makes their use an ideal approach to classifying samples obtained from ongoing, long‐term ecological monitoring programs.

Louisiana↗